Obviously the sand surface needs to be painted if it is to look like anything other than sand. These are large areas, and the sand has a large surface area as well as being abrasive. I use cheap acrylic paints and a cheap, stiff paintbrush - the stuff sold for kids is fine for this job. Colours are mixed from white, black, red, green, and yellow - I can't say I find mixing colours easy and I usually end up with something too dark. Also a single colour sometimes looks too flat and plain, the aim is for a finish that is not too even which I attempt with multiple washes.
The platform got a coat of pale grey, this seems to have come out well and looks fine as well tended gravel so I left it at that. The road and car park behind got a darker coat which looked too even, so it was followed by a much thinner coat of mid brown to weather it and add shadows. However, this looked too dark, so another thinned coat of pale grey was added. The result is a mid grey that isn't too even as the thinner coats have had differing effects at different depths.